Virology

1. Virology General Properties of Viruses: 1- They are very small in size, from 20-300 m. 2- They contain one kind of nucleic acid (RNA or DNA) as their genome. 3- They are metabolically inert because they do not possess enzyme systems necessary for the synthesis of new viral material (ribosomes). 4- They are obligate intracellular parasites as they replicate inside living cells. 5- They are only seen by electron microscope. 6- Depend on the parasitized cell for survival and multiplication. Dr. Rania Alhady

2. Viral Structure Viral Structure: Each virus particle or virion is composed of : • A protein coat capsid • A nucleic acid core • Many viruses are naked but some are enveloped. • Viral nucleic acid: Viruses contain a single type of nucleic acid either DNA or RNA but not both, that encodes genetic information necessary for viral replication. Most DNA viruses are double stranded, some are single stranded. Most RNA viruses are single stranded, some are double stranded. Functions of viral nucleic acid: 1- Site of genetic material that carries the heritable characteristics of the virus. 2 - Essential infective component of the virion. Dr. Rania Alhady

3. Viral Structure (2) Viral capsid: It is the protein coat surrounding the nucleic acid. It is composed of small protein subunits arranged symmetrically around the nucleic acid called capsomeres. Functions of capsid: Protection of nucleic acid from damage in external environment. Facilitates attachment of the virus to the host cell in process of infection. Site of viral antigen. Dr. Rania Alhady

4. Viral Structure (3) Viral envelope: Many viruses are surrounded by a lipid or lipoprotein envelopes which may be covered by spikes (glycoproteins). Enveloped viruses are sensitive to lipid solvents, e.g. ether and chloroform. N.B. The nucleic acid of the virus surrounded with its protein coat is called nucleocapsid. Dr. Rania Alhady

5. Virus Symmetry Virus Symmetry: Viruses have three types of symmetry:Cubical symmetry: These viruses resemble a crystal and are called icosahedral virus. Example: adenoviruses. Helical symmetry: In which the particle is elongated. Most helical viruses are enveloped Example: influenza virus. Complex symmetry: In which the viruses are complicated in structure. Example: poxviruses and bacteriophage. Dr. Rania Alhady

6. Virology Classification of Viruses: Classical virus classification schemes have been based on the consideration of major properties of viruses: 1- The type of nucleic acid which is found in the virion (RNA or DNA, singlestranded or double stranded). 2- The symmetry and shape of the capsid (Cubic, helical, complex). 3- The presence or absence of an envelope (enveloped, naked) 4- The size of the virus particle. 5- Antigenic properties: susceptibility to physical and chemical agents. 6- Biologic properties, including natural host range, mode of transmission, vectorrelationship and pathogenicity. Dr. Rania Alhady

7. Virology DNA Classification of Viruses: Icosahedral Complex Naked Enveloped Hepatitis B Adenoviruses Herpes Lecture #6 Dr. Rania Alhady

8. Virology: 8 Dr. Rania Alhady

9. Viral Replication Viral Replication: Viruses multiply only in living cells. Steps of viral replication: 1- Attachment (Adsorption): Virus attaches to specific receptors on the cell surface. 2- Penetration (Entry or Engulfment): Non-enveloped viruses: penetrate host cell by process of pinocytosis or phagocytosis forming cytoplasmic vacuole that dissolves and then release nucleocapsid into the cytoplasm. Enveloped viruses: penetrate host cell by process of endocytosis (fusion of viral envelope with the cell membrane), thus releasing nucleocapsid directly into the cytoplasm. 3- Uncoating: Uncoating is the physical separation of viral nucleic acid from the outer structural components (capsid). Done in cytoplasm by cell enzymes. The infectivity of virus lost at this point. Dr. Rania Alhady

10. Viral Replication 4- Eclipse phase: Most of virus particles can not be detected for some hours. The cellular metabolism is now directed towards the synthesis of new virus particles. This phase ends with formation of the infectious virus. 5- Synthesis of viral components: The essential step in virus replication is transcription of mRNA from the viral nucleic acid. Once it is accomplished, viruses use cell components (ribosomes and tRNA) to translate the mRNA into different viral components. This involves the synthesis of viral proteins (enzymes, capsid) and viral genomes. 6- Assembly: New virus particles are assembled by packaging of the genome into capsid. 7- Release: Virus may be released to the extracellular environment due to cell lysis, or, if enveloped, may bud from the cell leaving the host cell intact. Dr. Rania Alhady

11. Viral Replication Dr. Rania Alhady

12. Cytopathic Effects Effect of viruses on cells: Many viruses inhibit host RNA, DNA or protein synthesis (or any combination of these). In general, viruses could induce the following pathogenic effects: 1- Cytopathic effect (CPE): The presence of the virus often gives rise to morphological changes in the host cell. These may include: 1- Membrane blebbing 2- Formation of multinucleated giant cells, or fused cells, known as syncytia 3- Production of inclusion bodies (accumulations of viral proteins or virions) in the nucleus or cytoplasm 4- Rounding up and detachment of cells from the culture dish 5- Cell lysis Dr. Rania Alhady

13. Cytopathic Effects 2- Latency: Virus genome persists within the infected cells without replication and the survival of infected cells. There will flare-ups of clinical disease. 3-Chromosome damage: As breakage, fragmentation, rearrangement, translocation, inversion and deletion. 4- Malignant transformation (oncogenesis): The host cells acquire new characters that lead to rapid growth and mostly to malignancy. Viruses are known to be the etiologic factors in the development of human tumors. Examples of oncogenic viruses include: - Human papillomavirus (HPV) : Cervix Cancer - Ebstein Barr virus (EB) : Nasopharyngeal carcinoma - Hepatitis B and Hepatitis C viruses (HBV, HCV) : liver Cancer - Human T cell lymphotorpic virus (HTLV): Adult T cell leukemia. Dr. Rania Alhady

14. CytopathicEffects Cytopathic effects: Left to Right: Cytopathic effect of HSV, enterovirus 71, and RSV in cell culture. Note the ballooning of cells in the cases of HSV and enterovirus 71. Note syncytia formation in the case of RSV Dr. Rania Alhady

15. Virology Viral Infection may be: (1) Superficial:Short incubation period without invasion e.g.respiratory viruses, influenza virus and Rota virus. (2) Systemic:Viruses enter the body by inhalation, ingestion, or parentral route. Virus spread to local lymphoid tissue then invasion of regional lymph nodes followed by transient viraemia, resulting in seeding of the virus to target organs where replication may occur. e.g. - Skin and mucous membrane, e.g.measles&chicken pox. - Brain, e.g. encephalitis (arboviruses). - Liver, e.g. hepatitis (hepatitis virus A, B, C). (3) Persistent:may be: A- Chronic infection: detection of virus with mild symptoms (HBV). B- Latent infection:Herpes virus. C- Slow infection: very long IP with no clinical symptoms, e.g.HIV Dr. Rania Alhady

16. Virology Cultivation and Assay of Viruses: A) Embryonated chicken eggs: Inoculation of chorioallantoic membrane, amniotic sac, allantoic sac or the embryo itself. Embryonated eggs are better than animal inoculation because: -Sterile. -Having no immunological function. -Easily available and inexpensive. B) Tissue culture: This is the most widely used. Types of cells: 1- Malignant cell lines “continuous cell lines”, e.g. human carcinoma of cervix (Hela cells). 2- Monkey kidney cells (primary cell culture). 3- Human embryonic fibroblasts (secondary cell culture). C) Animal inoculation: Inoculation of virus into the animal leads to certain pathological effects. The infected tissues are then obtained, from which virus suspension can be prepared. Dr. Rania Alhady

17. Virology Dr. Rania Alhady